Dutch elm disease is a fungal disease affecting elm trees, caused by fungi of the genus Ophiostoma. The disease is carried in the form of fungal spores by bark beetles that infest the inner layer of tree bark. Fungi deposited in the tree infect its vascular system, often leading to the death of the tree. The disease originated in Asia and spread to Europe and the United States in the early 20th century. Managing Dutch elm disease is a matter of interrupting its cycle, often by using insecticide to discourage beetle infestation or by administering fungicides to the tree to combat infection.
Elm bark beetles are often responsible for the spread of Dutch elm disease. If a beetle lay eggs in a tree infected with the fungus that causes the disease, its larvae will become carriers when they hatch and begin consuming parts of the tree. When the larvae become adults, they may visit other elm trees and deposit fungi while eating the inner bark of the new tree. Trees may also become infected when an infected tree's root system fuses with that of another, adjacent tree.
Once disease-causing fungi have been introduced to the tree, they move up and down within the tree via a type of transport tissue called xylem. The fungi settle around the roots of the elm tree within a few months and begin growing up the tree trunk. Trees with Dutch elm disease usually have wilting leaves that eventually turn yellow and then brown. As the withering leaves can no longer provide nutrients for the tree through photosynthesis, over time the roots of the tree starve and die.
Dutch elm disease reached Europe around 1910, but this early, mild strain of the disease did not greatly affect the elms of the continent. The disease was isolated and identified in the Netherlands in 1921 by Bea Schwarz, hence the name "Dutch elm disease." In 1928, the disease was brought to the United States, probably in a shipment of logs from the Netherlands. It spread through New England and traveled westward across the country, devastating many different elm species. The disease then spread to Canada around the time of World War II, severely decimating the Canadian elm population.
Disease management can be as simple as removing dead or withering branches from existing elm trees and cutting down dead trees when necessary. Caring for the trees in this way can keep Dutch elm disease under control. Insecticides may also be used to deter bark beetles from infesting trees. Fungicide administered prior to infection or during the early stages of disease can protect the life of the trees. The spread of the disease can also be limited by planting tree varieties that have been engineered to resist the disease.